1. Field of the Invention
The invention relates to a compound and a method of preparing the same, and more particularly to tricobalt tetraoxide as a precursor of cathode materials for lithium-ion batteries and a preparation method thereof.
2. Description of the Related Art
In 1991, Sony Corp. (Japan) successfully developed a cathode material of lithium cobalt oxide for lithium-ion batteries. Since then, lithium-ion batteries have rapidly developed in industry and have been widely used in mobile phones, cameras, notebook computers, and portable electrical appliances.
Due to obvious advantages in resources, labors, and policies, lithium-ion battery industries have developed rapidly in China and have attained a strong international competitiveness. Battery-grade tricobalt tetraoxide (Co3O4) is one of the key materials for the preparation of the cathode material of lithium cobalt oxide. With the increasing demand on lithium ion rechargeable batteries, the demand on Co3O4 has also been increased. Compared with conventional Co3O4 for industrial use, battery-grade Co3O4 demands higher purity, and has stricter requirement on physical properties (such as density, particle size, specific surface area, particle size distribution, and crystal morphology), electrochemical performance, and stability.
Chinese Pat. No. 03148092.6 disclosed a method of preparation of high purity and spherical tricobalt tetraoxide, and Chinese Pat. No. 200510015002.9 disclosed a method of preparation of an anode material of tricobalt tetraoxide for lithium ion batteries.
Conventional methods of preparation of tricobalt tetraoxide include gas phase methods, liquid phase methods, and solid phase methods. Powder Co3O4 is generally prepared by a combustion method or pyrolysis method. However, powder Co3O4 obtained by a combustion method has a low purity, large particle size, wide particle size distribution, and poor sintering activity, and its physical and chemical properties hardly meet the requirements of the electronic industry. A pyrolysis method is to decompose a divalent cobalt compound, such as CoAc2, CoC2O4, Co(OH)2, or CoCO3 under a certain temperature to yield Co3O4. However, the method consumes a large amount of energy and the resultant Co3O4 has a poor activity.
A method of preparation of monodisperse nano-sized powdery Co3O4 by a hydrothermal method is known, but the method is not suitable for industrial production.
A method of preparation of spherical powdery Co3O4 having good dispersive property by controlling the kind and using amount of precipitant and surfactant is also know. The precipitant used was NH4HCO3, and high molecular weight of surfactant was involved, which caused difficulty in filtering the precipitate.
A method of preparation of Co3O4 by directly oxidating a mixture consisting of Co(NO3)2, H2O2, and NaOH in aqueous solution is also known. But the resultant precipitate contained cobalt hydroxide and hydrates thereof, and when dried and calcined, an aggregate consisting of particles (0.5 μm) was obtained. Avoiding aggregate formation in preparation of powdery Co3O4 became a problem.
A method of preparation of Co3O4 using Na2CO3 as a precipitant is also known. However, the resultant precipitate was hardly separated, and impurity such as Na+ adsorbed on colloidal particles was hardly removed by washing. If NH4HCO3 is used as a precipitant, the resultant precipitate is basic cobalt carbonate. Although the precipitate was separated and impurities removed, Co3O4 was obtained only under high temperature, and its appearance was unsatisfactory. Furthermore, when NH4HCO3 was used as a precipitant for preparation of Co3O4, a large amount of energy was consumed, the precipitation was not complete, a large amount of metal was lost, and much wastewater containing ammonia was produced.